Thanks to advice of fellow Apple Discussions member Brandon Jenkins, I have been able to put together a guide to using a Sony Ericsson P900 with Apple's iSync and Address Book
Please download the PDF which contains the necessary steps, plus the zip file which contains the necessary files to enable the P900 and iSync.

DOWNLOAD: PDF Guide: updated 23 July 2004.
DOWNLOAD: Files necessary for P900 under 10.3.4/iSync Guide

For iSync 10.3.5 and iSync 1.5:

It appears that Apple have repaired most of the issues with the p900 in the iSync 1.5 update. If you have just bought a P900 and already have 10.3.5 and iSync 1.5 on your Mac, you shouldn't need to make any modifications. However, if you upgraded to 10.3.5 and iSync 1.5 and are still having problems, download the updated files below and use the guide (above). People who already followed the guide under 10.3.4 & iSync 1.4 may also need to download and apply the new files:

DOWNLOAD: Files necessary for P900 under 10.3.5 and iSync 1.5

Please also check out Brandon's recommendations at the Apple Discussion Forum and the latest Apple Discussions (go to the iSync section) for information and news:

"Okay folks. I decided to start a new thread instead of posting in another as I can without a doubt say that the following works as I have helped numerous folks on a myriad of configurations. Please read through and if you have any questions post within.

Softwares used:
OS X 10.3.5
iSync 1.5
Organiser R5B02

With 10.3.4 iSync required some hacking of plist files to get working. Those files and steps have been well documented by Simon Spence. http://homepage.mac.com/sspence/p900/

Numerous challenges face the unsuspecting Apple user in syncing these SE phones. Most notably an understanding how the sync process works. I have identified what are the 3 issues to date and work arounds for each.

After Apple released both it's 10.3.5 update and 1.5 iSync update I decided I would test the waters with an "out of the box" experience. I backed up all data and reinstalled the OS as it came from Apple. Afterwards, I applied all patches including the 10.3.5 and iSync 1.5 updates. Upon finishing I attempted to pair my device with the Mac and it found services for Address Book and GPRS but not iSync. The addition of Address Book was new with the fixes applied in 10.3.5 and iSync. After doing a quick search I stumbled upon an apple link which indicated adding the device through iSync would work, and it did.

Upon attempts to sync failing I resorted to a few known tricks.

1) Turn on the address book bluetooth connection. As stated by Andrea Colombo mRouter is broken in 10.3.4 and still seems to be so in 10.3.5. The connection just can't seem to get going unless it is helped along by another application. I highly recommend Salling's Clicker for this. I use it daily and it maintains a persistent connection with the phone and will reattach itself after the sync is done.

2) As I had done a Master Reset on the phone as part of the test, iCal entries would not sync over until I opened the calendar app on the phone for the default files to be created. I believe this is a known issue as well.

3) Address book integration is not optimal still. The Address Book plist file needs to be updated to support the commands that the phone will support. SMS is not supported. Apple, if you're reading this, please update Address Book.app appropriately. The information can be found in the ABDeviceCommandSets.plist file in Simon's files.

4) A custom firewall opening still needs to be done for iSync to work with the firewall on. A very good feature request would be for iSync to handle the opening and closing of the hole as it is attempting to sync. (Again, Apple, pretty please :-) ) And if you are running a 3rd party firewall don't forget to modify it.

I hope this write up confirms things for people. It's getting closer to usable."

Full discussion available here

Simon Spence / November 2004